New motherboard supports DDR5 and DDR4

can you use ddr5 in ddr4 slot   Dan can you connect pcie 40 in 30 slot

The problem is that DDR5 RAM is NOT compatible with DDR4 RAM, and vice versa: Reminder: DDR4 RAM Won't Fit Into DDR5 Slots and Vice

use XMP compatible memory and intall them on a motherboard that supports XMP DDR4DDR5dimmdimm slotdual channelmemory slotquad channelXMP  DDR5 can use densities of up to 64 Gb DRAMs in a single-die package , where DDR4 maxes out at 16 Gb DRAM in an SDP Higher device capacity

amazing link poseidon slot No The sticks are physically different DDR4 wont fit in a DDR5 slot and vice versa  DDR5 has about the same latency as DDR4 and DDR3 DDR5 octuples the maximum DIMM capacity from 64 GB to 512 GB DDR5 also